Soil acidity

Soil pH is a measure of the acidity or basicity of a soil. pH is defined as the negative logarithm (base 10) of the activity of hydronium ions (H+
or, more precisely, H
3O+
aq) in a solution. In soils, it is measured in a slurry of soil mixed with water (or a salt solution), and normally falls between 3 and 10, with 7 being neutral. Acid soils have a pH below 7 and alkaline soils have a pH above 7. Ultra-acidic soils (pH<3.5) and very strongly alkaline soils (pH>9) are rare.

Soil pH is considered a master variable in soils as it affects many chemical processes. It specifically affects plant nutrient availability by controlling the chemical forms of the different nutrients and influencing the chemical reactions they undergo. The optimum pH range for most plants is between 5.5 and 7.5; however, many plants have adapted to thrive at pH values outside this range.

The United States Department of Agriculture Natural Resources Conservation Service classifies soil pH ranges as follows:

Methods of determining pH include:

Precise, repeatable measures of soil pH are required for scientific research and monitoring. This generally entails laboratory analysis using a standard protocol; an example of such a protocol is that in the USDA Soil Survey Field and Laboratory Methods Manual. In this document the three-page protocol for soil pH measurement includes the following sections: Application; Summary of Method; Interferences; Safety; Equipment; Reagents; and Procedure.

The pH of a natural soil depends in the mineral composition of the parent material of the soil, and the weathering reactions undergone by that parent material. In warm, humid environments, soil acidification occurs (soil pH decreases) over time as the products of weathering are leached by the flow of water through the soil. In dry climates, however, soil weathering and leaching are less intense and soil pH is often neutral or alkaline.

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